‘Dark Pool’ Settlements Bring Tangled Relationships to Light

Banks quietly catered to high-speed traders in dark pools

The latest round of penalties over “dark pools” highlights how reliant banks and exchange operators have become on business from high-frequency traders—even on platforms that promised to blunt their advantage.

Dark pools were advertised to mutual funds and other traditional money managers as a place where they could trade in secret and avoid giving away their moves to computer-driven traders. New settlements with two of the biggest dark-pool operators, Credit Suisse Group AG and Barclays PLC, showed that the banks were quietly...